This invention relates to a communications system comprising a plurality of communications modules and at least one control module, in which each station is in communicating contact with the controller either directly or indirectly. Communications systems of this type may be used to form a number of different types of systems for example: a security system for buildings, motor vehicles or the like: a security system for ensuring that data which is transmitted within a data transmission system is secure from external influences: a system for allowing data from cash tills in a supermarket to communicate with a central stock control unit, etc.
Known communication systems of this type comprise a control module, and a plurality of communications modules spaced apart from the control module and from one another. If the communications system is being used as a security system for a building, for example, each communications module will be positioned at vulnerable parts of the building, for example at window latches and door locks. The control module may be positioned at any convenient position. In such a known communications system, the control module communicates directly with each station in turn, and there is no communication possible between neighbouring stations.
A problem with this known communication system is that the power of the control module must be sufficient to allow transmission between the control module and the most distant of the communications modules. For many applications it is necessary to have at least some of the communications modules positioned a long distance from the control module. This results in the control module having to have a high power which adds to the cost of the system, and also results in the system having to fulfill certain licencing requirements. High power systems have to pass stringent design and manufacturing standards, the user has to purchase a licence periodically, and in certain cases has to pass a test on its proper use. The licence is relatively expensive, the reason is that, if your system interferes with one other user per square mile, then the greater the range, (the higher the power) the greater the amount amount of interference you may cause. Also the greater the range you wish to receive over, the greater are your chances of being interfered with by other users. Doubling the range gives rise to an eight fold increase in the likelihood of interference. In addition as unnecessarily high power demand will be required which often cannot be conveniently supported by batteries even when it can be supported by batteries, the modules have either a very short useful life, or are very large (and expensive) and detract from the benefits of using "free standing" RF modules by being unecessarily cumbersome.
Another problem associated with known communications systems, is that it is relatively east for an intervener to listen to the messages sent by the control module to a particular communications module, and then to disable the particular module by substituting a substitute module in the place of the original module. This enable someone trying to penetrate the system to disable a particular module, and gain access to the system. For example, if the system, is being used as a building security system, by disabling a particular module, access to the building at that point may be gained. If the substitute station is capable of transmitting and receiving exactly as the original station, the control module will not detect that anything is wrong.
Another problem in known communications systems is that a communications module which has received data from a second communications module is required to send a message to the transmitting module on receipt of the data, to confirm that the data has been received. This reduces the inherent speed of the system.